The present invention relates to precoding matrix design and, more particularly, to precoding matrix design to derive a precoding matrix as a product of two matrices.
Wireless communication systems demand for even higher spectral efficiencies to accommodate the higher throughput requirements within the limited frequency bands. Multiple antenna or multiple-input and multiple-output (MIMO) systems and in particular closed loop transmission technologies such as beamforming and precoding have been vastly considered to improve the spectral efficiency. In MIMO precoding schemes, the data to be transmitted is divided into one or more streams, the streams are mapped onto one or more transmission layers, and the data in the layers are precoded with a precoder or precoding matrix before transmission. The number of transmission layers is called transmission rank. The transmission rank can be optimally chosen for a given channel realization by considering, for example, the transmit power and the overall channel statistics.
In codebook based precoding strategies, a predetermined codebook is made available to the transmitter, i.e., base station (BS), and all receivers, i.e., mobile stations (MSs) or user equipments (UEs). The receiver then chooses a precoder from the codebook which maximizes its performance (e.g. its data rate) and feeds back the precoder index. The selection of precoder rank may also be included in the precoder selection algorithm. The feedback rate may vary from a short-term feedback once every coherent time interval to a long-term feedback once every several coherent time intervals.
In many systems, the optimal precoders from the codebook for two adjacent transmission blocks are close with respect to a proper distance measure in the set of all possible precoders. Here, the adjacent blocks may be considered in time or in frequency, e.g., over the set of tones in orthogonal frequency-division multiplexing (OFDM) systems since in practical systems the channel does not change abruptly from one transmission block to the adjacent one. Thus, the precoder used in those blocks can be equal if the channel is pretty steady and the codebook resolution is not too high. By increasing the codebook resolution or having a more dynamic channel, the precoders of the adjacent blocks are not equal anymore, yet, they might be close. The closeness between two precoders can be measured based on a proper distance metric in the space of all such precoders. Some examples of differential, dual and multi-resolution codebooks are disclosed in refs. [5] and [6].
We consider precoding codebook design for the 4 transmit antenna (TX) MIMO downlink channel and detail a codebook structure that is suitable for both the uniform linear array (ULA) and the cross-pole antenna configurations, in order to obtain a codebook that is efficient, i.e., has a low feedback overhead and is easy to store and search over, and effective over both uniform linear array (ULA) and the cross-pole configurations. Some have proposed codebook designs for specific antenna configurations [7]. The fundamental properties of the spatial correlation matrices that we use have not been exploited in the prior art. The codebook structure herein is derived using fundamental properties of the spatial correlation matrices under the ULA and cross-pole antenna configurations. Each precoding codeword is derived as the product of two matrices which makes them efficient and achieves lower feedback overhead for a given performance level and better performance for a given feedback overhead.